In fossil-fired power plants, a series of methods for separating carbon dioxide from the exhaust gas of the power plant are currently being developed and tested. In this case, scrubbing methods are employed, in which carbon dioxide is separated from the exhaust gas by chemical or physical absorption prior to combustion (precombustion) or after combustion (post combustion). In the post-combustion method, carbon dioxide is absorbed in absorption columns by a scrubbing agent which is regenerated again in a desorption column following the absorption column, thereby releasing carbon dioxide. The regenerated scrubbing agent is subsequently conducted from the desorption column via corresponding countercurrent heat exchangers back to the absorption column again and is thus circulated. For the desorption process, a large quantity of heat energy is required for regenerating the scrubbing agent and is supplied to the desorption column via sump evaporators and via heat exchangers in side vents of the desorption column.
The main problem in the existing methods for separating carbon dioxide from an exhaust gas is, in particular, the very high energy outlay which is required in the form of heating energy for desorption. No useful improvements have hitherto been found in this regard in the prior art which sufficiently reduce the energy outlay of a separation device for separating carbon dioxide which is integrated into a power station process.
The necessary heat energy has hitherto been extracted in the form of heating steam from the power station process. In fossil power station processes, at the present time a large part of the low-pressure steam of the steam turbine, usually between 30 and 70%, is used for regenerating the scrubbing agent, thereby reducing the overall efficiency of the power plant by 4 to 7%. Moreover, if an existing power plant is appropriately retrofitted with a carbon dioxide separation device, this results in considerable conversion work on the steam and water circuits and on the steam turbines and consequently in considerable investment.
The general disadvantage of carbon dioxide separation devices which are known from the prior art is, in particular, the very high energy outlay. Furthermore, the diversion of steam from the power plant entails considerable additional investment and makes it difficult to operate the power plant in the various operating states. A further disadvantage arises particularly in the run-up phase of the power plant. The process steam for regenerating the scrubbing agent can be made available only when the power plant generates sufficient process steam. It therefore takes a longer time to transfer the separation plant into a stable separating operation, and, consequently, more carbon dioxide is discharged, unpurified, into the atmosphere.